Defense Secretary Mark Esper sits down with Fox News national security correspondent Jennifer Griffin for his first interview since taking over at the Pentagon.

China is the Pentagon's “number one priority,” and the United States is watching Beijing "very carefully" in order to safeguard America, Defense SecretaryMark Esper told Fox News in an exclusive interview Wednesday.

In his first interview since he was confirmed as Pentagon chief in July, Esper told Fox News national security correspondent Jennifer Griffin that China has engaged in the “greatest theft of intellectual property in human history” and is also expanding its military to “push the United States out of [the Indo-Pacific] theater.”

"China is the number one priority for this department. It’s outlined in the National Defense Strategy, why we think it’s a long-term strategic competitor and one that is pursuing a maximization campaign, if you will, throughout the Indo-Pacific Theater, whether it’s politically, economically, or militarily," he told Griffin. "They are clearly professionalizing and expanding the capacity and capabilities of the military in order to push the United States out of that theater."

“They’ve studied us, and they’ve learned about how we employ weapons; they’ve learned about our doctrine,” Esper added. “And so, that is something that we watch very carefully.”

Esper, a former Army lieutenant colonel, graduated from West Point in 1986 – the same class as Secretary of State Mike Pompeo – and spent 10 years on active duty, followed by 11 years in the Army National Guard and Reserve. He brings military, defense and national security experience to the table, and touts a “good relationship” with President Trump.

In his wide-ranging interview, Esper also spoke about deterring Russia’s nuclear ambitions, protecting American elections from foreign hackers, denuclearizing the Korean Peninsula and Iran funding its “illicit activities.”

Esper noted what he described as the coming shift from “low-intensity conflict that lasts 18 years,” referring to the war in Afghanistan, to “high-intensity conflicts against competitors such as Russia and China.”

“That means modernizing the force with advanced capabilities, A.I.-based, hypersonics, robotics, directed energy and updating our doctrine -- doing all those things will be critical for us to deter a conflict in the future,” he said.

Esper described China's theft of intellectual property as a “big, big problem.”

“It’s a state-run organized effort to go after technologies, whether they are defense or non-defense technologies, to go up against other -- all other types of intellectual property, even commercial goods,” he explained. “So, it really requires enforcement. And this is where […] I applaud the president for pushing back against China and all their trade activities that are outside the bounds of what should be expected.”

In addition to safeguarding intellectual property, Esper said the U.S. needs to modernize its nuclear stockpile.

“Our strategic forces are a key deterrent to nuclear war. I think a strong, reliable, capable, ready deterrent is really what prevents nuclear war from happening in the first place,” he pointed out.

Esper noted the Russians were “clearly […] trying to expand their strategic nuclear arsenal […] to deal with the United States.”

“And so, as people talk about a New Strategic Arms Reduction Treaty -- that’s why we say, look, if there’s going to be an extension of New START, then we need to make sure we include all of these new weapons that […] Russia is pursuing,” he stressed.

Russian President Vladimir Putin has warned his country would develop short- and intermediate-range land-launched nuclear missiles if it got word the United States had started building such weapons.

When it came to Russian hacking, Esper said the U.S. will see “continued malign Russian cyber activity.”

“We will see North Korean cyber activity and Chinese cyber activity,” he went on. “But I will say, we built exquisite capabilities ourselves under Cyber Command. And I think it’s one of the reasons why, between our capabilities and between the authorities granted to us by President Trump in the last year or so, we had little problems with 2018 elections."

“[We] will continue to apply all of our capabilities and all of our authorities to make sure that we -- our elections -- are protected and that the integrity of our democracy is unquestionable,” Esper noted.

The defense secretary also talked about President Trump’s proposed Space Force, and why it was needed – he pointed to nations like Russia and China, “probably more so China,” which are turning space into a “war-fighting domain.”

"Ten years ago, space was a place -- the heavens upon which we looked down upon the Earth and figured out what was going to be the weather in Iowa, or we could survey our adversaries,” he explained.

But, since then, China and Russia have been working to overtake U.S. systems in space and threaten not only the U.S. military but the country’s economy and commerce, as well, Esper said.

“We want to be able to develop a space force that would build the space capabilities in a coherent […] fashion, but, at the same time, have a Space Command that would be responsible for the space war-fight,” he said.

The Pentagon chief also told Fox News he applauded the president’s approach to North Korea, saying diplomacy was the way to achieve denuclearization of the Korean Peninsula “in a verifiable, complete, and irreversible way.”

Griffin later asked Esper about former Defense Secretary Jim Mattis, who resigned from his post in December after disagreeing with the president about the possibility of pulling troops from Syria.

“That was his red line,” Griffin said. “What is your red line?”

“So look, what I said is my issue with any person I ever worked for was, I would never do anything that is either illegal, immoral or unethical. And, I don’t believe the president’s going to ask me to do any one of those three,” Esper said.

Griffin also asked Esper about a moniker in his West Point yearbook.

“In your yearbook, they call you Troop. Why’d they call you Troop?” she asked. Mattis famously had the nicknames “Mad Dog,” “Warrior Wonk” and “Chaos.”

“Because I was really, really committed to military service and being in infantry, and that was kind of a nickname I inherited by a couple friends,” Esper said.

Griffin also asked Esper if anything kept him up at night and referenced claims by Trump that nothing keeps him up at night.